The Pima Indian population is characterized by a strikingly high prevalence of obesity and by the highest recorded prevalence of diabetes in the world. A group of non-diabetic obese Indians were compared to a group of Caucasians who were matched for age, degree of obesity, sex, and glucose tolerance status. The obese Caucasians were then further matched by a group of lean Caucasinas of similar age, sex, and glucose tolerance status. Obese subjects of both races had reduced insulin sensitivity as assessed by the euglycemic clamp technique. There was no significant racial difference. The kinetics of insulin distribution and destruction was not influenced in a major way by race or obesity. Both alpha cells and beta cells of the islets of Langerhans were inhibited by the induced hyperinsulinemia. The plasma glucagon levels remained suppressed long after the plasma insulin levels had returned to basal. Plasma C-peptide reactivity however showed a more rapid recovery from insulin suppression.